Early automated library systems consisted of a handful of storage cells, a simple robot, and one or two media drives. Each storage cell held one data cartridge. Each media drive accommodated one data cartridge at a time. The robot's function was to manipulate the data cartridges one at a time, moving them between the various storage cells and the media drives. One common automated library system design arranged the storage cells and media drives in a two-dimensional array. The robot for this arrangement was movable along straight lines in X and Y directions across the face of the array. Another common design arranged the storage cells and media drives in a cylindrical pattern. Here, the robot was moveable along and around the cylinder's axis.
Complexity and performance of the robots increased as the number of data cartridges and media drives within the robotic storage libraries increased. Eventually though, the robot's performance became a limiting factor in the overall performance of the automated library system. One robot could only mount or dismount one data cartridge at a time from one media drive. This limitation was overcome by connecting multiple copies of the automated library systems together with interlibrary transfer mechanisms. Now, multiple robots could operate simultaneously to service multiple media drives simultaneously.
Continued increases in the number of data cartridges and media drives revealed that the interlibrary transfer mechanisms were the next limiting factor. Interlibrary transfers required the attention of the robots in both the sending library and the receiving library. While the robots were busy transferring a data cartridge, they could not service the media drives.
The latest generation of automated library systems use multiple rail guided robots. Each rail guided robot is free to move among all of the storage cells and media drives simultaneously. This allows multiple media drives to be serviced simultaneously. Another advantage is that the rails may be routed between adjacent libraries effectively merging all of the libraries into one large library system. Dedicated interlibrary transfer mechanisms and their associated limitations are unnecessary.
Rail guided robots raise issues concerning power distribution. Power was easily provided to the earlier robots. Slip rings transferred power to robots mounted on a fixed axis of rotation. Flexible wiring harnesses carried power to robots that moved back and forth along straight paths. In contrast, rail guided robots have no fixed mounting location and can move along complex paths. Slip rings do not work with rail-based systems, and flexible wiring harnesses can become entangled as two or more rail guided robots cross each other's paths. Rail guided robots must receive power through a fixed distribution medium and/or have an onboard power source.
Power distribution through rails is commonly used in railroad technologies. Large railroad locomotives receive three-phase electrical power through collector shoes engaging power rails paralleling the train tracks. Model railroads use a similar type approach by distributing single-phase or direct current electrical power through the train tracks themselves to conductive wheels on the railroad locomotives.
Both railroad power distribution methods are potential particulate contamination generators due to wear on the contacting surfaces. Intermittent bouncing between the contacting surfaces may also generate considerable electromagnetic noise. Neither of these situations is desired in the automated library environment. Particulate contamination can interfere with information transfers between the data cartridges and media drives. Electromagnetic noise can corrupt the information.
Onboard power sources for autonomous robots within an automated library system are disclosed in U.S. Pat. No. 5,395,199 issued to Day, III et al. on Mar. 7, 1995. Day's robots are powered by onboard batteries. Batteries do not generate particulate contaminants, nor do they generate electromagnetic noise. However, batteries require occasional recharging making the robot unavailable for extended periods.
Rail guided robots also raise issues concerning power scaling. Power scaling for rail guided robots with onboard batteries require scaling of the number of installed battery chargers. Additional battery chargers must be added to the automated library system as additional rail guided robots are added to keep all of the batteries charged. Attention must also be given to the layout of the rails so that each rail guided robot can reach one of the battery chargers when its onboard battery runs low.
Rail transmitted power scaling requires tradeoffs between the maximum current capacity of each rail and the number of independent power zones within the automated library system. Rail size can be kept reasonable by limiting the maximum wattage or number of rail guided robots that can draw power from any one rail at any given time. This usually requires large automated library systems to be divided into multiple power zones.
Rail guided robots moving between adjacent power zones and switching between adjoining rails may be required to accommodate potential differences in the electrical power in each power zone and possibly a powerless region. In a make-before-break type transition, the rail guided robots must be able to connect to two adjacent power zones simultaneously without severe interactions. In a break-before-make type transition, the rail guided robots must have some minimal onboard power source to sustain themselves while they are out of contact with all power zones.